As shown in FIG. 26, a dental air turbine handpiece 120 includes a grip part 121 and a head part 122 provided at a tip portion of the grip part 121. An operator performs a cutting operation on teeth while gripping the grip part 121, for example.
Conventionally, there has been known a bearing unit 100 for a dental air turbine configuring the head part 122. As shown in FIG. 27, the head part 122 includes a rotary shaft 101 having one end to which a dental treatment tool (not shown) is to be attached, a turbine blade 102 configured to rotate the rotary shaft 101 by compressed air, and a pair of ball bearings 103, 104 configured to rotatably support the rotary shaft 101. The rotary shaft 101, the turbine blade 102 and the pair of ball bearings 103, 104 are disposed in a head housing 105. Outer rings 106, 107 of the pair of ball bearings 103, 104 are supported via rubber rings 108 mounted in annular concave portions 109, 110 of the head housing 105. The outer ring 107 of the lower ball bearing 104 is urged upwardly by a spring washer 111, so that the pair of ball bearings 103, 104 is applied with preload. That is, the pair of ball bearings 103, 104 configured to support the rotary shaft 101 is combined in a face-to-face arrangement and is applied with positive preload by an elastic force of the spring washer 111.
In the dental air turbine handpiece 120, the ultrahigh-speed rotation is implemented by enabling the compressed air to flow against the turbine blade 102 and thus rotating the rotary shaft 101 by the air pressure.
Patent Document 1 discloses a dental or medical air turbine handpiece configured by a head housing having an air supply opening, a rotary shaft rotatably accommodated in the head housing and including a blade configured to receive the supply air from the air supply opening, and ball bearings configured to vertically rotatably support the rotary shaft within the head housing with the air supply opening being interposed therebetween. Herein, a heat resistance mechanical seal is provided at a side of the ball bearing close to the air supply opening.
Therefore, when a pressure of the supply air acts in the air supply opening, i.e., when the blade is rotating, the heat resistance mechanical seal is elastically deformed to contact the ball bearing and shields the pressure. On the other hand, when the pressure of the supply air is not applied, i.e., when the rotation of the blade is stationary, the heat resistance mechanical seal returns to an original state so as to be in non-contact with the ball bearing. Thereby, it is possible to prevent a situation where lubricant oil filled in the ball bearing is leaked by the pressure of the supply air applied into the ball bearing during usage. Further, when the rotation of the blade stops, the heat resistance mechanical seal and the ball bearing are made not to contact each other, so that a frictional resistance between the heat resistance mechanical seal and the blade is removed upon activation of the blade of the rotary shaft and the blade can be thus smoothly activated.